Before the introduction of ES6 classes, JavaScript developers primarily used closures and constructor functions to create object factories, each with distinct behaviors and implications for encapsulation. While closures naturally support encapsulation by hiding private data behind public APIs, JavaScript classes lack this feature, although a proposal to add it is in progress. The traditional method of using underscores to denote private properties in classes is problematic due to the risk of unintended data modification by both new and experienced developers. In terms of implementation, classes leverage the `this` keyword and use the `new` keyword for instantiation, whereas closures do not rely on `this` and are instantiated by calling the function. Classes often perform faster as they share a prototype among instances, making them more memory efficient for multiple instances, while closures create unique references in memory for each instance, which aids encapsulation but is less efficient. The choice between using closures or classes depends on the project's requirements, with closures offering simplicity for single-instance use and classes providing efficiency in multi-instance scenarios.